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Elsevier's Integrated Physiology E-Book

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Each title in the new Integrated series focuses on the core knowledge in a specific basic science discipline, while linking that information to related concepts from other disciplines. Case-based questions at the end of each chapter enable you to gauge your mastery of the material, and a color-coded format allows you to quickly find the specific guidance you need. Bonus STUDENT CONSULT access - included with the text - allows you to conveniently access the book's content online · clip content to your handheld device · link to content in other STUDENT CONSULT titles · and more! These concise and user-friendly references provide crucial guidance for the early years of medical training, as well as for exam preparation.
  • Includes case-based questions at the end of each chapter
  • Features a colour-coded format to facilitate quick reference and promote effective retention
  • Offers access to STUDENT CONSULT! At www.studentconsult.com, you'll find the complete text and illustrations of the book online, fully searchable · "Integration Links" to bonus content in other STUDENT CONSULT titles · content clipping for handheld devices · an interactive community center with a wealth of additional resources · and much more!

Subjects

Books
Savoirs
Medicine
Epidermis (anatomía)
Derecho de autor
Realimentación
Riñón
Gripe
Ácido desoxirribonucleico
Editorial
Puberty
Sickle-cell disease
Myocardial infarction
Influenza
Photocopier
Humulin
Volume contraction
Emphysema
Clinical Medicine
Cell physiology
Renal blood flow
Cardiovascular physiology
Osmolarity
Actin-binding protein
Dermatitis
Respiratory alkalosis
Male reproductive system (human)
Second messenger system
Pregnancy
Protein S
Metabolic acidosis
Weight gain
Muscle contraction
Book review
Gestational diabetes
Pericarditis
Vascular resistance
Human musculoskeletal system
Renal function
Physical examination
Blood flow
Iron deficiency anemia
Dihydrotestosterone
Pulmonology
Hemolytic anemia
Review
Hereditary spherocytosis
Hypotension
Alkalosis
Acidosis
Carbon monoxide poisoning
Myosin
Extracellular fluid
Physician assistant
Actin
Pulmonary edema
Glycemic index
Addison's disease
Epidermis
Steroid hormone
Follicle-stimulating hormone
Arterial blood gas
Heart failure
Fibrin
Androgen
Meeting
Mentorship
Atmosphere of Earth
Alopecia
Cough
Physical exercise
Urinary incontinence
Infertility
Growth hormone
Embryology
Dehydration
Acetylcholine receptor
Bleeding
Shock (circulatory)
Permeability
Atherosclerosis
Anemia
Hypertension
Electrocardiography
Anaphylaxis
Human gastrointestinal tract
Testicle
Peptic ulcer
Coeliac disease
Integumentary system
Obesity
Urinary system
Diarrhea
Endometriosis
Philadelphia
Melanin
Menopause
Blood vessel
Asthma
Kidney stone
Infection
Lung
Artery
Data storage device
Salicylic acid
Salinity
Pharmacology
Physiology
Pediatrics
Estrogen
Neuroscience
Mechanics
Molecule
Lipid
Kidney
Immunology
Insulin
Erectile dysfunction
Haematopoiesis
Histology
Hypoglycemia
Feedback
Food
Email
Endocrine system
Dentistry
Carbon dioxide
Biochemistry
Urushiol
Hypertension artérielle
Divine Insanity
Ménopause
Pneumothorax
Feed
Jogging
Consultant
Éditorial
Release
Cortisone
Mentor
Body Fluids
Ovulation
Fatigue
Electronic
Progestérone
Estradiol
Testostérone
Hypotension artérielle
Contact
Acid
Constipation
Rétroaction
Maladie infectieuse
Philadelphie
DNA
Grippe
Surface
Syncope
Neurosciences
Copyright
Molécule
1
Air
Adénosine triphosphate
Glucose
Hormone
Plasma

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Published 04 December 2006
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EAN13 9780323082914
Language English
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Elsevier’s Integrated
Physiology
Robert G. Carroll, PhD
Professor of Physiology, Brody School of Medicine, East Carolina University, Greenville,
North CarolinaTable of Contents
Cover image
Title page
Copyright
Dedication
Preface
Editorial Review Board
Series Preface
Chapter 1: Physiology: The Regulation of Normal Body Function
PHYSIOLOGY
LEVELS OF ORGANIZATION
COMMON THEMES
APPLICATION OF COMMON THEMES: PHYSIOLOGY OF
THERMOREGULATION
TOP 5 TAKE-HOME POINTS
Chapter 2: The Integument
EPITHELIA
INTEGUMENT LAYERS
ROLE OF SKIN IN THERMOREGULATION
CUTANEOUS GROWTH AND REGENERATION
VITAMIN D PRODUCTION
IMMUNE FUNCTIONTOP 5 TAKE-HOME POINTS
Chapter 3: Body Fluid Distribution
BARRIERS BETWEEN COMPARTMENTS
MEASUREMENT OF BODY FLUID COMPARTMENTS
MOVEMENT ACROSS BARRIERS
BODY FLUID AND ELECTROLYTE BALANCE
TOP 5 TAKE-HOME POINTS
Chapter 4: Cellular Function
CELL STRUCTURE AND FUNCTION
CELL DEVELOPMENT
CELL-TO-CELL COMMUNICATION
CELL MEMBRANE
CELL ELECTRICAL ACTIVITY
MODULATION OF TISSUE RESPONSE TO LIGAND
TOP 5 TAKE-HOME POINTS
Chapter 5: Musculoskeletal System
STRUCTURE OF SKELETAL MUSCLE
SKELETAL MUSCLE TYPES
SMOOTH MUSCLE
CARDIAC MUSCLE
SKELETON
TOP 5 TAKE-HOME POINTS
Chapter 6: Blood and Hematopoiesis
HEMATOPOIESIS
BLOOD COMPONENTS
HEMOSTASIS
FIBRINOLYSIS AND ANTICOAGULANTS
TOP 5 TAKE-HOME POINTSChapter 7: The Heart
STRUCTURE OF THE HEART
CARDIAC ELECTROPHYSIOLOGY
MYOCARDIAL PHYSIOLOGY
CARDIAC MECHANICAL ACTIVITY
NEURAL AND HORMONAL REGULATION OF THE HEART
TOP 5 TAKE-HOME POINTS
Chapter 8: Vascular System
BLOOD VESSEL HISTOLOGY
VASCULAR SEGMENTS
HEMODYNAMICS
MICROCIRCULATION
NEURAL AND HORMONAL REGULATION OF VASCULATURE
CIRCULATION IN SPECIFIC VASCULAR BEDS
TOP 5 TAKE-HOME POINTS
Chapter 9: Integrated Cardiovascular Function
CONCEPTUAL MODEL OF CARDIOVASCULAR INTEGRATION
REGULATION
ARTERIAL HYPOTENSION AND SHOCK
CARDIAC AND VASCULAR FUNCTION CURVES
EFFECTS OF RESPIRATION ON CARDIOVASCULAR FUNCTION
EFFECTS OF ACCELERATION AND GRAVITY ON CARDIOVASCULAR
FUNCTION
INTEGRATION AND REDUNDANCY OF CARDIOVASCULAR CONTROL
CARDIOVASCULAR ADJUSTMENT TO EXERCISE
TOP 5 TAKE-HOME POINTS
Chapter 10: Pulmonary System
PULMONARY SYSTEM PHYSIOLOGY MAPSTRUCTURE AND FUNCTION OF THE RESPIRATORY SYSTEM
VENTILATION
SURFACTANT AND PULMONARY COMPLIANCE
WORK OF RESPIRATION
GAS EXCHANGE
PULMONARY CIRCULATION
VENTILATION-PERFUSION BALANCE
BLOOD TRANSPORT OF OXYGEN AND CARBON DIOXIDE
REGULATION OF PULMONARY FUNCTION
TOP 5 TAKE-HOME POINTS
Chapter 11: Renal System and Urinary Tract
RENAL SYSTEM STRUCTURES
FUNCTION OF THE ELIMINATION SYSTEM
RENAL TUBULAR SEGMENTS
RENAL HANDLING OF WATER AND ELECTROLYTES
URINARY CONCENTRATION AND DILUTION
URINARY ACID-BASE REGULATION
REGULATION OF RENAL FUNCTION
EXCRETION
RENAL ENDOCRINE FUNCTION
RENAL METABOLIC FUNCTION
EFFECTS OF NUTRITION
TOP 5 TAKE-HOME POINTS
Chapter 12: Gastrointestinal System
GASTROINTESTINAL MOTILITY
GASTROINTESTINAL SECRETIONS
DIGESTION AND ABSORPTION
TOP 5 TAKE-HOME POINTSChapter 13: Endocrine System
ENDOCRINE SYSTEM FUNCTION
ENDOCRINE SYSTEM REGULATION
HYPOTHALAMUS AND PITUITARY
THYROID
PARATHYROID HORMONE AND CALCITONIN
ADRENAL GLAND
ENDOCRINE PANCREAS
RENIN-ANGIOTENSIN SYSTEM AND ATRIAL NATRIURETIC PEPTIDES
TOP 5 TAKE-HOME POINTS
Chapter 14: Female Reproductive System
SEXUAL DIFFERENTIATION IN UTERO
PUBERTY
ADULT FEMALE REPRODUCTIVE STRUCTURES
BREASTS
FEMALE REPRODUCTION
TOP 5 TAKE-HOME POINTS
Chapter 15: Male Reproductive System
SEXUAL DIFFERENTIATION IN UTERO
PUBERTY
ADULT MALE REPRODUCTIVE STRUCTURES
INTERCOURSE AND ORGASM
TOP 5 TAKE-HOME POINTS
Chapter 16: Life Span
PREGNANCY
NEONATAL PHYSIOLOGY
GROWTH
TOP 5 TAKE-HOME POINTSChapter 17: Integration
INTEGRATED ACID-BASE REGULATION
EXERCISE
TOP 5 TAKE-HOME POINTS
Case Studies
Case Study Answers
IndexC o p y r i g h t
1600 John F. Kennedy Blvd
Suite 1800
Philadelphia, PA 19103-2899
ELSEVIER’S INTEGRATED PHYSIOLOGY
ISBN-13: 978-0-323-04318-2
ISBN-10: 0-323-04318-6
Copyright © 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical,
including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Permissions may be sought directly from Elsevier’s Health Sciences Rights Department in Philadelphia, PA, USA: phone: (+1) 215 239
3804, fax: (+1) 215 239 3805, e-mail: healthpermissions@elsevier.com. You may also complete your request on-line via the Elsevier
homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’.
Notice
Knowledge and best practice in this field are constantly changing. As new research and experience broaden our knowledge, changes in
practice, treatment and drug therapy may become necessary or appropriate. Readers are advised to check the most current information
provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or
formula, the method and duration of administration, and contraindications. It is the responsibility of the practitioner, relying on their
own experience and knowledge of the patient, to make diagnoses, to determine dosages and the best treatment for each individual
patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor the Author assumes
any liability for any injury and/or damage to persons or property arising out or related to any use of the material contained in this
book.
The Publisher
Library of Congress Cataloging-in-Publication Data
Elsevier’s integrated physiology.
p. cm.
ISBN 0-323-04318-6
1. Human physiology.
QP34.5.E47 2007
612—dc22
2006043013
Acquisitions Editor: Alex Stibbe
Developmental Editor: Andrew Hall
Printed in China
Last digit is the print number: 9 8 7 6 5 4 3 2 1D e d i c a t i o n
In memory of my friend and mentor, the late Dr. David F. Opdyke, and with many
thanks to my teachers at the University of Medicine and Dentistry of New Jersey–
Newark.
Preface
At a conference, I was asked to summarize physiology in twenty- ve words or less.
Here is my response: “The body consists of barriers and compartments. Life exists
because the body creates and maintains gradients. Physiology is the study of
movement across the barriers.” Twenty-five words exactly.
This book is organized along those lines. Most chapters begin with an
anatomic/histologic presentation of the system. Function does indeed follow form,
and the structure provides limitations on physiology of a system. Physiology,
however, is the study of anatomy in action. If anatomy is the study of the body in
three dimensions, physiologic function and regulation extend the study of the body
into the fourth dimension, time.
Robert G. Carroll, PhDEditorial Review Board
Chief Series Advisor
J. Hurley Myers, PhD
Professor Emeritus of Physiology and Medicine, Southern Illinois University School of
Medicine
President and CEO, DxR Development Group, Inc., Carbondale, Illinois
Anatomy and Embryology
Thomas R. Gest, PhD, University of Michigan Medical School, Division of
Anatomical Sciences, Office of Medical Education, Ann Arbor, Michigan
Biochemistry
John W. Baynes, MS, PhD, Graduate Science Research Center, University of
South Carolina, Columbia, South Carolina
Marek Dominiczak, MD, PhD, FRCPath, FRCP(Glas), Clinical Biochemistry
Service, NHS Greater Glasgow and Clyde, Gartnavel General Hospital, Glasgow,
United Kingdom
Clinical Medicine
Ted O’Connell, MD
Clinical Instructor, David Geffen School of Medicine, UCLA
Program Director, Woodland Hills Family Medicine Residency Program, Woodland
Hills, California
Genetics
Neil E. Lamb, PhD
Director of Educational Outreach, Hudson Alpha Institute for Biotechnology, Huntsville,
Alabama
Adjunct Professor, Department of Human Genetics, Emory University, Atlanta, Georgia
Histology
Leslie P. Gartner, PhD, Professor of Anatomy, Department of Biomedical
Sciences, Baltimore College of Dental Surgery, Dental School, University of
Maryland at Baltimore, Baltimore, Maryland
James L. Hiatt, PhD, Professor Emeritus, Department of Biomedical Sciences,
Baltimore College of Dental Surgery, Dental School, University of Maryland at
Baltimore, Baltimore, Maryland
Immunology
Darren G. Woodside, PhD, Principal Scientist, Drug Discovery, Encysive
Pharmaceuticals Inc., Houston, Texas
MicrobiologyRichard C. Hunt, MA, PhD
Professor of Pathology, Microbiology, and Immunology
Director of the Biomedical Sciences Graduate Program, Department of Pathology and
Microbiology, University of South Carolina School of Medicine, Columbia, South
Carolina
Neuroscience
Cristian Stefan, MD, Associate Professor, Department of Cell Biology,
University of Massachusetts Medical School, Worcester, Massachusetts
Pharmacology
Michael M. White, PhD, Professor, Department of Pharmacology and
Physiology, Drexel University College of Medicine, Philadelphia, Pennsylvania
Physiology
Joel Michael, PhD, Department of Molecular Biophysics and Physiology, Rush
Medical College, Chicago, Illinois
Pathology
Peter G. Anderson, DVM, PhD, Professor and Director of Pathology
Undergraduate Education, Department of Pathology, University of Alabama at
Birmingham, Birmingham, Alabama5
Series Preface
How to Use This Book
The idea for Elsevier’s Integrated Series came about at a seminar on the USMLE Step 1 exam at an American Medical Student Association
(AMSA) meeting. We noticed that the discussion between faculty and students focused on how the exams were becoming increasingly
integrated—with case scenarios and questions often combining two or three science disciplines. The students were clearly concerned about
how they could best integrate their basic science knowledge.
One faculty member gave some interesting advice: “read through your textbook in, say, biochemistry, and every time you come across a
section that mentions a concept or piece of information relating to another basic science—for example, immunology—highlight that section
in the book. Then go to your immunology textbook and look up this information, and make sure you have a good understanding of it. When
you have, go back to your biochemistry textbook and carry on reading.”
This was a great suggestion—if only students had the time, and all of the books necessary at hand, to do it! At Elsevier we thought long and
hard about a way of simplifying this process, and eventually the idea for Elsevier’s Integrated Series was born.
The series centers on the concept of the integration box. These boxes occur throughout the text whenever a link to another basic science is
relevant. They’re easy to spot in the text—with their color-coded headings and logos. Each box contains a title for the integration topic and
then a brief summary of the topic. The information is complete in itself—you probably won’t have to go to any other sources—and you have
the basic knowledge to use as a foundation if you want to expand your knowledge of the topic.
You can use this book in two ways. First, as a review book …
When you are using the book for review, the integration boxes will jog your memory on topics you have already covered. You’ll be able to
reassure yourself that you can identify the link, and you can quickly compare your knowledge of the topic with the summary in the box. The
integration boxes might highlight gaps in your knowledge, and then you can use them to determine what topics you need to cover in more
detail.
Second, the book can be used as a short text to have at hand while you are taking your course …
You may come across an integration box that deals with a topic you haven’t covered yet, and this will ensure that you’re one step ahead in
identifying the links to other subjects (especially useful if you’re working on a PBL exercise). On a simpler level, the links in the boxes to
other sciences and to clinical medicine will help you see clearly the relevance of the basic science topic you are studying. You may already be
confident in the subject matter of many of the integration boxes, so they will serve as helpful reminders.
At the back of the book we have included case study questions relating to each chapter so that you can test yourself as you work your way
through the book.
Online Version
An online version of the book is available on our Student Consult site. Use of this site is free to anyone who has bought the printed book.
Please see the inside front cover for full details on the Student Consult and how to access the electronic version of this book.
In addition to containing USMLE test questions, fully searchable text, and an image bank, the Student Consult site o ers additional
integration links, both to the other books in Elsevier’s Integrated Series and to other key Elsevier textbooks.
Books in Elsevier’s Integrated Series
The nine books in the series cover all of the basic sciences. The more books you buy in the series, the more links are made accessible across
the series, both in print and online.
Anatomy and Embryology
Histology
Neuroscience
Biochemistry
Physiology Pathology
Immunology and Microbiology
Pharmacology
Genetics2
The Integument
CONTENTS
EPITHELIA
INTEGUMENT LAYERS
Epidermis
Dermis
Hypodermis
ROLE OF SKIN IN THERMOREGULATION
Sensory Reception
CUTANEOUS GROWTH AND REGENERATION
VITAMIN D PRODUCTION
IMMUNE FUNCTION
TOP 5 TAKE-HOME POINTS
EPITHELIA
Epithelial cells provide a continuous barrier between the internal and external environments. Epithelial cells line the skin, sweat glands,
gastrointestinal (GI) tract, pulmonary airways, renal tubules, and pancreatic and hepatic ducts. Consequently, materials in the GI lumen,
respiratory airways, renal tubules, reproductive system lumens, and secretory ducts are functionally “outside” the body. Compounds that are
secreted across epithelia are exocrine secretions, in contrast to endocrine secretions, which remain within the body. For example, pancreatic
digestive enzymes that are secreted into the lumen of the small intestine are exocrine pancreatic secretions, in contrast to insulin and
glucagon, which are secreted into the blood as endocrine pancreatic secretions.
Epithelial cells have polarity, since the tight junctions between cells separate the epithelial cell membrane into an apical and a basolateral
surface (Fig. 2-1). Epithelial tight junctions allow osmotic and electrochemical gradients to exist across the epithelia. The apical surface faces
the outside of the body or, for the GI tract and secretory ducts, a lumen. The basal and lateral surfaces face the inside of the body, or serosa,
and are surrounded by extracellular + uid. Epithelia express di, erent populations of protein transporters on the apical surface and the
basolateral surface. The structural integrity of epithelial cells is provided by tight junctions and by desmosomes, a site of attachment for the
extracellular matrix protein keratin.FIGURE 2-1 Tight junctions separate the apical membrane from the basolateral membrane of the epithelial cell. The
proteins expressed on the apical membrane differ from the proteins on the basolateral surface, providing polarity or
orientation for epithelia. The epithelial cell barrier allows the concentration of compounds on one side of the epithelium to
be different from the concentration of that compound on the other side of the epithelium.
Epithelia are specialized to serve a variety of functions. Epithelia provide a physical barrier, often supplemented by epithelial cell
secretions. Lipids and keratin in the skin provide a waterproof barrier. Mucous secretions protect the GI, female reproductive, and lung
epithelia from abrasive damage. Cilia of the respiratory and fallopian tube epithelia move mucus and + uid lining the epithelia toward the
mouth or vagina, respectively, for expulsion. Some epithelial cells are specialized for transepithelial transport of ions, nutrients, and
metabolic wastes.
Epithelial membranes contain specialized transport proteins. These proteins promote absorption of nutrients into the body from luminal or
duct contents, and secretion into luminal or duct + uids for excretion from the body. The common functional role of epithelia is re+ ected in the
common transport proteins located in apparently different organs. Identical sodium-dependent amino acid and glucose transporters are found
−in the epithelia of the small intestine and renal proximal tubule. Identical Cl reabsorbing channels are found in epithelia of salivary glands,
sweat glands, pancreatic ducts, and bile ducts. Genetic defects in these transport proteins a, ect all organs that express the protein. For
−example, defects in the Cl channel cystic 1brosis transmembrane regulator (CFTR) a, ect the lungs, exocrine pancreas, sweat glands, and GI
tract.
Transit across the epithelial barrier occurs by two pathways—transcellular and paracellular (Fig. 2-2). Transcellular transport passes
through the cell and consequently has to cross both the apical and basolateral membranes. Carrier proteins are necessary to move
lipidinsoluble substances across these cell membranes. Vesicular movement, such as pinocytosis, may be necessary for larger proteins. Paracellular
movement occurs through the tight junctions and water-1lled spaces between cells. This is the primary pathway for water-soluble substances
in some epithelia.FIGURE 2-2 Transepithelial absorption can go across the epithelial cells in the transcellular pathway or between the
epithelial cells in a paracellular pathway. Compounds absorbed by the transcellular pathway have to cross both the apical
and the basolateral membranes and travel through the cytoplasm of the cell. Movement through the paracellular pathway
is determined by the permeability of the tight junctions that join the epithelial cells.
Transepithelial water movement occurs in response to an osmotic gradient. Movement of the solvent water causes a change in theconcentration of the solutes on either side of the epithelia—solute concentration increases on the side where the water exits, and solute
concentration decreases on the side where the water enters. If the tight junctions are also permeable to the solute, water movement can cause
solute movement, a process called solvent drag.
Paracellular movement is restricted by the “tightness” of epithelial tight junctions. “Tight” tight junctions restrict the paracellular
movement of water and electrolytes. “Loose” tight junctions allow the paracellular movement of water and electrolytes. Tight junction
permeability varies between tissues and within different regions of the same tissue. Water-impermeable areas include the esophagus, stomach,
and portions of the renal tubules distal to the loop of Henle. Water-permeable areas include the small intestine and renal proximal tubules.
Transport of ions across epithelium generates a transepithelial potential. This electrical force may oppose further movement of ions,
analogous to the membrane potential. Transepithelial potential is important for aldosterone action in the renal distal tubule and connecting
segment (see Chapter 11) (Fig. 2-3).FIGURE 2-3 Impermeable epithelial tight junctions are necessary to develop a significant transepithelial electrical
potential. The reabsorption or secretion of ions across epithelia can establish electrical charge differences across the
epithelial barrier. Leakage of ions through the paracellular pathway can dissipate the electrical charge. If the tight junctions
are impermeable to ion movement, electrical potential will be maintained.
PATHOLOGY
Cystic Fibrosis
−Cystic 1brosis is a recessive genetic defect in the epithelial CFTR Cl channel. Cystic 1brosis occurs in approximately one of every 3500
−live births, and an estimated 10 million Americans are carriers of the defective gene. The impaired Cl movement interferes with
transepithelial water movement, resulting in excessively thick secretions that block the lungs, GI tract, and pancreatic and bile ducts.=
INTEGUMENT LAYERS
The integument, or skin, is the largest organ containing epithelial cells. Skin diminishes or prevents damage from trauma. The epidermal
layer provides a mechanical barrier, supplemented by cushioning by the adipose in the hypodermis. Bacteria, foreign matter, other
organisms, and chemicals penetrate it with di culty. Melanin in the epidermal layer diminishes damage from sunlight. The oily and slightly
acidic secretions of skin sebaceous glands protect the body further by limiting the growth of many organisms.
HISTOLOGY
Tight Junctions
Tight junctions regulate the movement of compounds through the paracellular pathway. Tight junctions are composed of the integral
membrane protein occludin and the extracellular family of claudin proteins. Tight junction size and charge permeability variations are due
to heterogeneity of the claudin proteins, which then determine the degree of “tightness” or “leakiness.”
Skin is impermeable to water and electrolytes, and it limits the transcutaneous loss of these compounds. Insensible loss of water and
electrolytes occurs only through pores. Burns and other injuries that damage the skin eliminate this protection and cause severe dehydration.
Skin makes up 15% to 20% of body weight. Skin has three primary layers: the epidermis, the dermis, and the hypodermis. Numerous
specialized structures are located in the epidermis, including eccrine glands, apocrine glands, sebaceous glands, hair follicles, and nails (Fig.
2-4).FIGURE 2-4 Skin comprises the superficial epidermal layer, internal dermal layer, and underlying hypodermal layer. The
hair, nails, and glands of the skin are extensions of the epidermis and penetrate deep into the dermal layer.
Epidermis
The epidermis is the thin, strati1ed outer skin layer extending downward to the subepidermal basement membrane. The thickness of the
epidermis ranges from 0.04 mm on the eyelids to 1.6 mm on the palms and soles. Keratinocytes are the principal cells of the epidermis, andproduce keratin. The cells replicate in the basal cell layer and migrate upward toward the skin surface. On the surface, they are sloughed o,
or lost by abrasion. Thus, the epidermis constantly regenerates itself, providing a tough keratinized barrier.
Skin coloration is due to both epidermal pigment accumulation and blood + ow. The primary cutaneous pigment is melanin, synthesized in
granules in epidermal melanocytes and a corresponding layer of the hair follicles. Skin color di, erences result from the size and quantity of
granules as well as from the rate of melanin production. Natives of equatorial Africa have an increase in the size and number of granules as
well as increased melanin production. In natives of northern Europe, the granules are small and aggregated, producing less melanin. With
chronic sun exposure, there is an increase in concentration of melanocytes as well as in size and functional activity. The presence of melanin
limits the penetration of sun rays into the skin and protects against sunburn and development of ultraviolet light−induced skin carcinomas.
Melanin that is produced in the epidermis can be deposited in the dermal skin layer through various processes (such as inflammation).
Melanocyte-stimulating hormone (MSH) is the primary controller of regulated melanin production. ACTH shares some sequence homology
with MSH, so high ACTH can cause melanin production and increase skin pigmentation, such as in Cushing’s disease (see Chapter 13).
Blood + ow to skin also imparts a tint re+ ecting the concentration and oxygenation of hemoglobin in the blood. Normally, oxygenated
hemoglobin imparts a pinkish/reddish color. Severe restriction of cutaneous blood + ow causes a whitish color, such as shock states. The
presence of deoxygenated hemoglobin causes a bluish color. These colors may not be apparent in skin regions with high melanin content but
can be seen in areas of relatively low melanin content such as the bed of the fingernail.
The epithelial barrier function is supplemented by hair and nails and secretions from sebaceous glands, eccrine glands, and apocrine sweat
glands. These structures are invaginations of epidermis into the dermis.
Nails and Hair
Nails and hair consist of keratinized and, therefore, “dead” cells. Nails are horny scales of epidermis that grow from the nail matrix at the
proximal nail bed. Fingernails grow about 0.1 mm/day, and complete reproduction takes 100 to 150 days. Toenails grow more slowly than do
1ngernails. A damaged nail matrix, which may result from trauma or aggressive manicuring, produces a distorted nail. Nails are also
sensitive to physiologic changes; for instance, they grow more slowly in cold weather and during periods of illness (Fig. 2-5).FIGURE 2-5 Hair and sebaceous gland secretions exit the epidermis at the hair follicle, whereas sweat glands exit by way
of independent ducts.
Hair is found on all skin surfaces except the palms and soles. Each hair follicle functions as an independent unit and goes through
intermittent stages of development and activity. Hair develops from the mitotic activity of the hair bulb. Hair form (straight or curly) depends
on the shape of the hair in cross-section. Straight hair has a round cross-section; curly hair has an oval or ribbon-like cross-section. Curved
follicles also affect the curliness of hair. Melanocytes in the bulb determine hair color.
HISTOLOGY
Hair Follicles
Hair follicles usually occur with sebaceous glands, and together they form a pilosebaceous unit. Sebaceous glands secrete + uid and lipids
into the hair follicle ducts, which act as waterproo1ng. Sebaceous gland secretion is enhanced by androgen secretion at puberty Arrector
pili muscles of the dermis attach to hair follicles and elevate the hairs when body temperature falls, producing “goose bumps.”
Epidermal Glands
Three di, erent types of glands are located on the epidermis. These glands are also composed of epithelial tissue; the glands themselves are
secretory epithelia, and the ducts leading to the surface of the skin have exchange epithelia.
Sebaceous glands are found throughout the skin and are most abundant on the face, scalp, upper back, and chest. They are associated withhair follicles that open onto the skin surface, where sebum (a mixture of sebaceous gland—produced lipids and epidermal cell—derived lipids)
is released. Sebum has a lubricating function and bactericidal activity. Androgen is responsible for sebaceous gland development. In utero
androgen causes neonatal acne; after puberty, increased androgen production again stimulates sebum production, often leading to acne in
adolescents.
Eccrine sweat glands play an important role in thermoregulation. They are found within most areas of the skin, but are particularly
numerous on the palms, soles, forehead, and axillae. Sweat is a dilute secretion derived from plasma. Eccrine gland secretion is stimulated by
heat as well as by exercise and emotional stress.
Apocrine glands secrete cholesterol and triglycerides and occur primarily in the axillae, breast areola, anogenital area, ear canals, and
eyelids. Sympathetic nerves stimulate apocrine secretion of a milky substance that becomes odoriferous when altered by skin surface bacteria.
Apocrine glands do not function until puberty, and they require high levels of sex steroids in order to function. In lower order animals,
apocrine secretions function as sexual attractants (pheromones), and the apocrine secretion musk is used as a perfume base. The role, if any,
in humans is not established.
Dermis
The dermis is a connective tissue layer that gives the skin most of its substance and structure. The dermoepithelial junction contains numerous
interdigitations that help anchor the dermis to the overlying epidermal layer. The papillary layer has loose connective tissue, mast cells,
leukocytes, and macrophages. The reticular dermis has denser connective tissue and fewer cells than does the papillary layer. The dermis has
a rich layer of blood and lymphatic vessels, including the arteriovenous anastomoses important in thermoregulation. The dermis also contains
numerous nerve endings, including a wide variety of the cutaneous sensory nerve receptors.
Hypodermis
The subcutaneous hypodermis layer is a specialized layer of connective tissue containing adipocytes. This layer is absent in some sites such as
the eyelids, scrotum, and areola. The depth of the subcutaneous fat layer varies between body regions and is based on the age, sex, and
nutritional status of the individual. Hypodermal adipose functions as insulation from extremes of hot and cold, as a cushion to trauma, and as
a source of energy and hormone metabolism.
ROLE OF SKIN IN THERMOREGULATION
Body temperature is maintained at 37°C as a result of balance between heat generation and heat loss processes. This balance involves
autonomic nervous system, metabolism, and behavioral responses. Even at rest, basal body metabolism generates an excess heat load that
must be dissipated to an environment that is usually cooler than 37°C. Heat loss across the skin can be controlled, and consequently the skin
plays a major role in the regulation of body temperature. Cutaneous participation in short-term thermoregulation involves blood + ow and
sweat production, part of complex process described in Chapter 1.
The dermal layer of the skin contains an extensive subcutaneous vascular plexus to assist in the regulation of body temperature (Fig. 2-6).
This plexus has an extensive sympathetic innervation, and an increase in cutaneous sympathetic activity constricts the blood vessels,
decreases cutaneous blood + ow, and consequently diminished heat transfer to the environment. The hypothalamus is partly responsible for
regulating adrenergic activity to the skin and therefore skin blood + ow, particularly to the extremities, the face, ears, and the tip of the nose.
Generally, the vessels dilate during warm temperatures and constrict during cold. Thermoregulation is assisted by countercurrent heat
exchange between arterial and venous blood flow in extremities.